Horse Activity Responses to an Elevated Temperature Humidity Index in the Arid Southwest

by Duarte E Diaz, Colt W Knight, Ashley D Wright, Pablo C Grijalva and Elizabeth A Greene*

Abstract: In Arizona, where high temperatures (>32°C) are typical, there are limited data on horse behavior, activity, and voluntary use of shade structures to block solar radiation as temperatures rise. This study aimed to examine if increasing Temperature Humidity Index (THI) affected the amount of time horses spent near a shade structure during daylight hours and/or decreased overall activity. From June through August 2017, four paddocks with existing small shade structures at the UArizona Equine Center were utilized to determine equine voluntary shade usage. Two groups of horses were fitted with global positioning system tracking collars, and animals’ locations were recorded daily for 15 hours at 5-second intervals (0500-2000). Each group spent one collection period (two days adaptation, five days data collection) in each paddock. Variations in heat and humidity were normalized by calculating THI for each hour of the collection period, ranging from 62.7 to 85.5 (averaging 78.3). Horses were considered near the shade when they were within 15m of the center of the shadow cast by shade. Overall, horses spent little time near the shade (average 8%) during data collection hours, and shade use behavior was not correlated with hourly THI changes, or distance traveled (activity). Horses elected to spend more time close to supplemental feeding areas than shade structures. In conclusion, horse activity to seek shade was not affected by increasing THI at levels observed in this study; however, paddock orientation, water trough location, and supplemental feeding location played significant roles in overall shade usage.